One of the reasons for introducing semiconductor photonics in hardware industry is increasing the speed of data transmission by using photons instead of electrons. In semiconductor photonics, use of silicon is customary due to the low fabrication costs, high yield and high volume driver of already existing microelectronic manufacture plants. Instead of developing a new type of plant and workflow, the existing manufacturing methods are applied to semiconductor photonics, adapting as little as possible, for resources and time economy. This is one of the main reasons of the use of silicon.
While silicon and silicon nitride waveguides are becoming standard materials in photonic circuits, other materials are necessary for good circuit integration. One of the reasons for the need of other materials is the indirect bandgap of silicon, which results in thermal response, rather than photonic response, to an electric stimulus. Hence, laser sources normally need materials with direct bandgap, or at least with better photoemission than silicon. Appropriate materials for production of the active layer of an integrated laser source are the so-called III-V direct bandgap semiconductor materials (GaAs, InP, InGaAs, etc.). For other optical functions, such as optical modulators, III-V materials can be integrated on silicon photonic ICs. Hybrid integration of III-V direct bandgap semiconductor materials or III-V devices with silicon-based integrated circuits is therefore an important technology for obtaining a good photonic integrated circuit, within the industrial constrains of economy of time and resources. The integration strategy followed should provide good optical coupling between the III-V and silicon materials, taking into account the standards of the industry.
One of these industrial standards is the use of silicon-on-insulator (SOI), which presents several advantages, and decades of production have led to a highly optimized production and quality. SOI comprises an insulating thick layer (typically silicon oxide) covered by a thin layer of silicon. Currently, most publicly accessible foundries—more specifically foundries that produce photonic chips—produce photonic SOI device wafers with a silicon thickness of 220 nm. For obtaining good adiabatic coupling of light between a III-V waveguide structure and a silicon waveguide, one should be able to match the effective index of the mode propagating in the silicon and that in the III-V waveguide to allow optical coupling.
In FIG. 1, the effective indices of a waveguide fabricated in a 2 micrometer thick III-V layer stack 101, a 500 nm thick III-V layer stack 102, and a 200 nm thick III-V membrane 103 are represented as a function of their width. The straight lines represent the effective index of silicon slab waveguides for different silicon thickness. For a silicon thickness of 500 nm the effective index is given by the upper most dashed straight line 111, for a 400 nm thickness the effective index is given by the second dashed straight line 112, for a 300 nm thickness the effective index is given by the third dashed straight line 113, and for a thickness of 220 nm the effective index is given by the lowest straight dashed 114. The adiabatic taper coupler should be designed such that the effective index of the silicon waveguide and the effective index of the III-V waveguide cross, which happens for a standard silicon thickness of 220 nm and a 2 micrometer thick III-V waveguide at a width under 0.3 microns. Consequently, if one would wish to use a standard optical silicon platform having a silicon standard thickness of 220 nm, the corresponding III-V layer should be substantially narrower than 0.3 microns, as it can be seen in FIG. 1. This is difficult to realize in practice.
The process for obtaining good coupling hence requires non-standard manufacture of either silicon wafers or III-V waveguides, which is disadvantageous from an industrial point of view. This problem worsens in the case of the silicon nitride platform, where the waveguides have an even lower effective index.
In conclusion, it is desired to have an improved solution for the hybrid integration of III-V waveguide photonic components (such as lasers, amplifiers, modulators) on standard ‘thin’ silicon photonic platforms and on medium contrast photonic platforms such as silicon nitride based photonic platforms.